(Per)fluoropolyethers (PFPEs) and derivatives thereof are currently used in the lubrication of MRM, namely in the lubrication of sliding or moving parts of such media. In particular certain PFPE derivatives containing a PFPE chain having terminal and non-terminal polar functional groups have shown best performances; indeed, thanks to the high chemical stability of the PFPE chain and to the presence of polar functional groups, such derivatives are able to form an even, long-lasting lubricant film on the surface of the moving parts of MRM. In particular, magnetic disk drives surfaces are usually coated with a carbon overcoat to which polar functional groups ensure adhesion, thereby preventing the lubricant film from being spun off during revolution of the disk.
Examples of the above PFPE derivatives are those containing at least one phosphazene cyclic group and at least one PFPE chain bearing one or more hydroxy groups. The phosphazene cyclic group is highly stable from the thermal standpoint and further increases the stability of the PFPE chain; without being bound to theory, it is believed that the phosphazene ring acts as a Lewis base which counteracts the catalytic effect on thermal degradation of the PFPE due to the Lewis acids typically present as impurities in the MRM. KANG, H. J., et al. The Use of Cyclic Phosphazene Additives to Enchance the Performance of the Head/Disk Interface. Journal of the Society of Tribologists and Lubrication Engineering. March 199, p. 22-27. Instead, the one or more hydroxy groups ensure adherence to MRM surfaces.
Certain patent documents, including US 2002183211 A (AKADA TAMIO ET AL) 5 Dec. 2002, US 2008020171 A (MATSUMURA OIL RES CORP [JP]) 26 Jan. 2006, US 2008020171 A (MATSUMURA OIL RES CORP [JP]) 26 Jan. 2006, US 2008305975 A (SEAGATE TECHNOLOGY LLC [US]) 11 Dec. 2008, WO 2007/043450 (MATSUMURA OIL RES CORP [JP]) 19 Apr. 2007 and US 2012276417 A (WD MEDIA SINGAPORE PTE LTD [US]) 27 Jan. 2011 disclose lubricants for magnetic recording media containing at least one phosphazene cyclic group bearing at least one optionally substituted phenoxy group and at least one hydroxy-substituted perfluoropolyether chain.
However, it has been observed that the optionally substituted phenoxy group on the phosphazene ring might reduce the mobility of the lubricant and decrease the durability of the MRM. Thus, it would be desirable to provide PFPE phosphazene derivatives for the lubrication of MRM, which do not show this drawback; in particular, it would be desirable to provide PFPE cyclophosphazene derivatives wherein each phosphorous atom bears a PFPE chain having at least one —OH group.
US 2012251843 A (SEAGATE TECHNOLOGY LLC [US]) 4 Oct. 2012 discloses, inter alia, a composition comprising a central core comprising a cyclic group, including a cyclotriphosphazene, and six arms extending from the central core, wherein each arm comprises a PFPE or a PFPE derivative. The PFPE derivative can be a PFPE chain having one or more terminal groups including: —CH2OH, —OCH2CH2OH, —CH2CH(OH)CH2OH, —CH2CH(OH)CH3 and —OCH2CH2CH2OH. The PFPE derivative can also have non-terminal functional groups including —CH2CH(OH)CH2—, —CH(OH)CH(OH)CH2—, —CH2CH(OH)CH2CH(OH)CH2—, —CH(CH2OH)— and —CH(OH)CH(OH)CH2—. In particular, FIG. 7 illustrates a hexacyclophosphazene wherein each phosphorous atom bears two PFPE substituents of formula:—O—CH2CF2CF2O—Rf—OCF2CF2CH2OHwherein Rf is (CF2CF2CF2O)n with n equal to or higher than 3.
However, US 2012251843 does not teach how to manufacture the claimed composition.
US 2009318664 (SEAGATE TECHNOLOGY LLC) 24 Dec. 2009 discloses compounds comprising a backbone with a PFPE chain, wherein one or more cyclophosphazene rings is attached to or incorporated in the chain, and at least two functional groups attached either to the backbone or to the cyclophosphazene ring or both. This document reports one synthesis example disclosing the reaction of a compound referred to as A20H, having formula:
with epichlorohydrin, to provide and intermediate which is in turn reacted with a solution obtained after heating 6.7 mmol Fomblin® Z-DOL PFPE with 6.7 mmol potassium hydroxide in about 0.2 ml water. Therefore, in such solution, 50% of the hydroxyl groups equivalents of Fomblin® Z-DOL PFPE is in the salified form. This document does not contain examples teaching the synthesis of phosphazenes substituted with more than one PFPE chain, each chain bearing at least one hydroxyl group.
The synthesis of hexasubstituted cyclotriphosphazenes by reaction of a nucleophile (e.g. an amine, an alcohol or a phenol) with hexachlorophosphazenes is reported in the literature, for example in: ALLCOCK, H. R., et al. Phosphonitrilic Compounds. IV. Preparation and Polymerization of Allylaminophosphonitrile Compounds1,2. Journal Organic Chemistry. 1965, vol. 30, no. 3, p. 947-949. and in LEE, S. B., et al. Thermosensitive Cyclotriphosphazenes. J. Am. Chem. Soc. 2000, vol. 122, no. 34, p. 8315-8316. When the nucleophile is an alcohol or a phenol the corresponding alkaline metal alkoxide or phenoxide is required in order to accomplish the nucleophile substitution on the phosphorous atoms.
Reactions of chloro- or fluoro-cyclophosphazenes with alkoxides of aliphatic diols, for example with dilithiated propanediol, are also reported in the literature, for example in:
VELDBOER, K., et al. Liquid chromatography/elctrospray time-of-flight mass spectrometry for the characterisation of cyclic phosphazenes. Rapid Commun. Mass Spectrom. 2011, vol. 25, p. 147-154.
ANIL, Elias J., et al. Chemistry of diphenyltetrafluorophosphazene: Reactions with dilithiated diols. Journal of Fluorine Chemistry. 2006, vol. 127, no. 8, p. 1046-1053.
In greater detail, Anil et al., investigates on the reaction of gem-diphenyltetrafluorophosphazene [1,1-(C6H5)2]P3N3F4 of formula:
with dilithiated propanediol LiO(CH2)3OLi.
This reaction gives rise to complex product mixtures, typically containing four products:                a) spiro-{3,3-[O(CH2)3O]}[1,1-(C6H5)2]P3N3F2, having formula:        
                b) ansa-{3,5-[O(CH2)3O]}[1,1-(C6H5)2]P3N3F2, having formula:        
                c) bridged-[N3P3F3(C6H5)2][O(CH2)3O]N3P3F3(C6H5)2], having formula:        
and                d) dangling-[HO(CH2)3O](C6H5)2P3N3F3, having formula:        

In particular, at par. 4.3.1. of the experimental section, Anil et al. states that when pure dilithiated propanediol is used in the reaction, the resulting mixture contains 39% wt dangling-[HO(CH2)3O](C6H5)2P3N3F3, 10% wt bridged-[N3P3F3(C6H5)2][O(CH2)3O]N3P3F3(C6H5)2] and also 5% wt and 9% wt of the ansa- and spiro-products. At paragraph 4.3.2 of the experimental section, Anil et al. further teaches that a much higher amount of dangling-[HO(CH2)3O](C6H5)2P3N3F3 (71% wt), and only traces of spiro-, ansa- and bridged-products can be obtained using monolithiated 1,3-propanediol.
When the applicant tried to manufacture hexasubstituted PFPE phosphazene derivatives claimed in US 2012251843 by treatment of a perhalocyclophosphazene with a PFPE diol in accordance with the teaching of Anil et al., unsatisfactory results were obtained using a PFPE diol containing ≧50% wt of the corresponding alkoxide (i.e. a PFPE diol wherein at least one equivalent of alcohol groups was salified); indeed, the desired “dangling” phosphazene derivative, which can be schematically represented by the formula (1) here below:
wherein PFPE represents a (per)fluoropolyether chain and z represents an integer of 3 or morewas not only obtained in admixture with ansa-, spiro- and bridged-PFPE phosphazenes, which can be schematically respectively represented by formulae (2)-(4) below:
wherein PFPE and z are as defined above and w is selected from 0, 1 or 2,but the amount of bridged-PFPE phosphazene was relevant, usually higher than 50% wt, which resulted in multimodal molecular weight distributions thus increasing the polydispersity of the final product. Since narrow molecular weight distribution is an important requirement for MRM lubricants, the need was felt to provide a convenient method for the manufacture of PFPE phosphazene derivatives comprising a cyclic phosphazene wherein each phosphorus atom of the cyclic phosphazene bears two PFPE substituents, each one containing a PFPE chain having a chain end comprising at least one hydroxy group, said PFPE derivatives having a narrow molecular weight distribution.